The document reading apparatus, which reads a document by moving the document and scanning an image using a one-dimensional image sensor, requires that the document always moves at a constant speed in an area where the image is read.
In the document reading apparatus, two rollers facing each other pinch the document therebetween, and the document is fed into the apparatus one-by-one by rotating the rollers. The document is conveyed by rotating a pair of rollers, facing each other, which is arranged in the apparatus. The document fed into the apparatus is ejected to the outside of the apparatus by the rotating rollers, starting from a front edge of the document, as reading proceeds.
In case that the document is conveyed as the pair of rollers rotates, when a rear edge of the document separates from the pair of rollers, a gap, corresponding to the thickness of the document, temporarily occurs between the rollers.
As shown in FIG. 8, rollers 118, 119 arranged in an apparatus pinch and convey a document 136. Therefore, at least one roller 119 is pressed by a pressing means 135, like a spring. As shown in FIG. 9, when a rear edge of a document 142 separates from an area between the rollers 118, 119, a gap d temporary occurs. Then, the roller 119 with the pressing means 135 instantly moves a distance of the gap d, and comes into contact with the roller 118. Consequently, the rear edge of the document 142 is pressed and kicked out by the roller 119 to instantly move, and the position of the document 142 rapidly shifts.
When a roller arranged near a document reading device causes rapid shifting of the document separating from the roller, various troubles with image reading occur. For example, a straight line drawn on the document is mistaken for a curved line. In an image area where a gap between parallel lines is narrow, like a Chinese character, the gap is mistaken as being elongated, otherwise the parallel lines are mistaken for one line. Furthermore, when a color image is read out by resolving into three primary colors, color deviation occurs.
In particular, very precise image reading is frequently performed in these days. Therefore, even though speed of the document slightly varies when the document is read out, image quality is seriously deteriorated. The thicker the document becomes, the more serious the troubles of image reading become.
To solve the above-mentioned problems, Japanese granted patent publication No. 2931347 (Patent document 1) discloses a recording apparatus having a clearance between a paper pan and a release plate. The paper pan of the recording apparatus corresponds to a roller supporting member. The release plate corresponds to a member arranged in the apparatus body. In the apparatus, a part of the paper pan extends downwardly to form a rectangular projection, and a rectangular opening is made in the apparatus body. Since the opening is larger than the projection formed on a bottom plate, engagement between the opening and the projection, having a predetermined amount of clearance, is made.